Airfoil Pitching Moment and C.G. Location

[Don Cooper]

Do differing airfoil pitching moments typically have a significant effect on C.G. location? The question is based on the following. I have been experimenting with a series of aerobatic/semi-aerobatic IPS powered planes. The airfoil I picked for the next to last one had bad stall characteristics (verified after the fact with Profili). After struggling with it a while, I decided to build another one using the old standby Clark Y. I used the tail surfaces from the previous one and since the body was beat up, built another essentially identical body. The wing planform was identical, but with the Clark Y. Weights, to the accuracy of my scale, were the same for both versions (about 4 oz with everything but battery). When I tried to fly it this morning with the C.G. set at the same place as on the previous version, it took full power and lots of up elevator to nurse it around for one circle, never able to get more than 6 feet high. Then the storms came and I haven't been able to try again. According to Profili (again checked after the fact) the Clark Y has about twice as much negative moment as the previous airfoil. Is that enough to require a probably substantial rearward movement of the C.G., or is there something else I should check before flying again. I have made cursory checks of the thrust angle, and wing and tail incidences and they appear to be zero.

[Dave Robelen]

Hi Don,
You are quite correct in assuming that different airfoils have different pitching moments. Starting with a pitching moment ( a tendency for the wing to rotate in pitch, usually nose down) for a symetrical wing of zero, the moment becomes increasingly nose down as camber is added. A CG location of 25% would be common on models with symetrical wings, while it might take a CG of 40% or so to trim the wings with substantial undercamber.
I generally use a CG of 30-35% with a Clark Y as a starting point. The other factor in your case which might be an issue is the difference in maximum lift with the various airfoils. If your wing change resulted in a wing with less lift, that could lead you to fly in the condition you mentioned.
If it were simply nose heavy, there should still be enough power and lift to climb, even out of trim. I would also check the battery and motor to verify full peformance from both.
Regards, Dave

[Don Cooper]

Dave,
Thanks for the quick response. After a closer look at the plane, I have found three possible causes/contributors to the problem (1) the C.G. was at 28%, (2) while setting in my hot garage waiting for a decent day the horizontal tail had developed a warp resulting in a little negative decalage and (3) the elevator push rod was binding resulting in very limited 'up' being available. The Clark Y has a better lift curve slope than the other airfoil, which is why I was surprised when it didn't take off at a healthy climb angle. I will try a new motor and different battery next time.

[CalmAir]

Dave,

When using a symetrical airfoil wing, with full span ailerons, is it proper to include the aileron width to determine the chord dimension of the wing for figuring wing area and CG location?

Dale

[Dave Robelen]

Hello Dale,
In brief, yes, the ailerons should be included. The correct method of measuring area is to treat it as though the surface were casting a shadow on a flat surface below it from parallel light beams. The true span is taken after dihedral is added, and the section through the fuselage(s) is included. This applies to the tail surfaces as well: Stabilizer + elevator = Horizontal tail, etc.
Regards, Dave

[Don Cooper]

I fixed the warp and binding control rod and moved the C.G. back about 1/2 inch and tried again this morning (dead calm until about 7:30). It flew great. I think I have finally got the combination I have been looking for - fast and responsive enough to be fun, but not too fast and responsive for my skills and aged reflexes.

[CalmAir]

Okay Dave. Thanks.